How Smart Traffic Lights Work

One of the advantages of taking Electrical Engineering classes in college and having a degree in the subject is that I learned how a number of devices in our society work. The other day I saw someone who sent their children to jump up and down on a sensor used to detect if a car is overhead. In this case they were trying to open an arm for a parking lot but it is the same technology for traffic lights. Unfortunately this adult erroneously believed that traffic sensors are triggered by weight. I thought the same thing growing up. Imagine my surprise when I attended my Electromagnetic Physics class and the professor informed us that it is just a metal detector buried under the asphalt?

Before college I tried like those kids to get my bicycle to trigger traffic lights to turn green for me. After doing a number of bunny-hops with no luck, I would sheepishly walk over and push the pedestrian button. After college I learned a trick and can now trigger traffic signals to turn green for me about 80% of the time. The bike I currently use to commute has an aluminum frame. While aluminum is a great conductor of electricity, it isn't magnetic and that is what a metal detector is looking for. Iron is magnetic and also rather heavy so they try to use as little as possible of it in bicycles. There is one place they can't avoid it though and that is the bike chain. I have discovered that if you ride so your chain is directly over the sensor, there is enough iron in it to trigger the light. This may take a bit of trial and error for some lights but I have learned where all of the sensors are on my 3-mile ride to and from work each day.

When I saw those kids jumping up and down on the car sensor, I politely asked them to move and rode over the sensor. My trick worked and they entered the parking lot without any trouble. It is a simple trick but looks like magic to young kids.